fix issue that prevents 'internal' repositories being created in an E…

…nterprised Managed User organisation (integrations#781)

Error: POST https://api.github.com/orgs/ORG1/repos: 422 Public repositories are not permitted for Enterprise Managed Organizations. []

github/resource_github_repository.go

@@ -303,7 +303,7 @@ func resourceGithubRepositoryCreate(d *schema.ResourceData, meta interface{}) er
 
 	visibility, ok := d.Get("visibility").(string)
 	if ok {
-		if visibility == "private" {
+		if visibility == "private" || visibility == "internal" {
 			isPrivate = true
 		}
 	}